People kiss for the simple reason that it feels good. Our lips are full of sensitive nerve endings, which activate sensory areas in the brain.
人们接吻的原因很简单，就是它会让你感觉良好。我们的嘴唇布满了神经末梢，这些神经末梢连接着大脑相应的神经区域。
Kissing boosts the levels of neurotransmitter substances such as dopamine, serotonin, and oxytocin.
接吻使一些神经递质物质迅速增加，例如多巴胺，血清素以及催产素。
Dopamine triggers craving and desire, serotonin elevates a person's mood, and oxytocin is involved in forming emotional attachments.
多巴胺触发人们的欲望，血清素能使人们情绪激昂，催产素会对情绪的变化有影响。
Besides humans, the only animals that scientists have ever seen kissing are our closest ape relatives, chimpanzees, and bonobos, and even then it is a rare occurrence. So scientist think there must be a better explanation for it.
除了人类，科学家发现唯一有接吻行为的动物，就是我们的猿类近亲，黑猩猩和倭黑猩猩，即使在它们之中这种现象也极少发生。所以科学家们认为人类要接吻还有更好的理由。

Some scientists have theorized that kissing might help us assess a partner's immune system.
有科学家认为接吻可以获取对方免疫系统的信息。
When people kiss, they come close enough to sample the scent of each other's bodies. And that scent may contain clues to immune system compatibility.
当人们接吻时，相互间足够近的距离使他们可以感受到对方的气味。这些气味中包含着免疫系统兼容性的线索。
Parents with complementary immune systems have children with a stronger ability to resist disease. Children that are better at resisting disease are more likely to survive to have their own children.
具有互补性免疫系统父母的孩子更容易抵抗疾病。而这些抵抗疾病更强的孩子也能更好的生存下来并孕育健康的下一代。
Exchanging saliva may provide other chemical clues to genetic compatibility too. So, kissing really is all about chemistry!
交换唾液或许也提供了基因兼容所需的其它化学物质的一些线索。所以接吻真是一场化学反应！